Active laser remote sensing from space is considered an important step forward in the understanding of the processes which regulate weather and climate changes. The planned launching into polar orbit in the late 1990s of a series of dedicated Earth observation satellites offer new possibilities for flying lidar in space. Among the various lidar candidates, ESA has recognized in the backscattering lidar and Doppler wind lidar the instruments which can most contribute to the Earth observation program. To meet the schedule of the on-coming flight opportunities, ESA has been engaged over the past years in a preparatory program aimed to define the instruments and ensure timely availability of the critical components. This paper reviews the status of the ongoing developments and highlights the critical issues addressed.

Some of the major NASA programs using laser remote sensing techniques are summarized, with emphasis on the development of the Lidar In-space Technology Experiment and the Lidar Atmospheric Sensing Experiment. Insight is provided into the technology being developed for future experiments, primarily wavelength extension into the middle IR range of the spectrum, 3 to 5 microns, and for wind sensing and DIAL measurements. New metrology technology which will lead to improvement measurements for both earth sciences and space physics is discussed. Some of the developments in systems and technology for applications in aviation safety, microburst detection, clear air turbulence, and wave vortex detection are highlighted.

Progresses in remote sensing of the atmosphere using the light detection and ranging (lidar) technique closely follows progresses in laser technology. We developed a mobile differential absorption lidar (DIAL) system, based on high repetition rate excimer-pumped dye lasers, for performing 2-D and 3-D mappings of concentration of NO, NO₂, SO₂, and O₃. The high sensitivity of the system has been used for numerous environmental studies and measurement campaigns, providing for the first time a direct correlation between emission and immission. Attractive results have been obtained under urban conditions, because of the presence of strong concentration gradients, and fast fluctuations due to traffic. A comparative study between Lyon, France; Stuttgart, Germany; Geneva, Switzerland; and Berlin, Germany; is presented. In particular, the Berlin campaign demonstrates the possibility of detecting unknown emitters and monitoring exportation-importation processes of atmospheric pollution. A new stationary DIAL system has recently been constructed and implemented on the top of a building in the center of the city of Leipzig, Germany. It will routinely perform concentration mappings of nitrogen oxides, sulfur dioxide, and ozone, giving access to long term evolution of pollution distributions.

The application of a mobile differential absorption lidar (DIAL) system in monitoring the atmospheric distribution of atomic mercury is discussed. The DIAL technique using the 253.65 nm Hg resonance line has been employed in studies of industrial emissions as well as geophysical manifestations. Hg concentrations down to the background value 2 ng/m³ can be measured.

Laser remote sensing offers a great opportunity to measure immission and even emission of gaseous pollutants in the troposphere. Large areas can be 3-dimensionally mapped in a fairly short time, thus dynamic behaviors in terms of vertical or horizontal concentration distributions can be derived. ARGOS (advanced remote gaseous oxide sensor) is an industrial manufactured system based on a DAS-lidar (differential absorption and scattering) method combined with a SODAR (sound detecting and ranging) for wind profile measurements. ARGOS has been presented at an SPIE conference in 1990 with the main emphasis on requirement specifications and realization. Field measurements of tropospheric sulfur dioxide and ozone have been conducted successfully and are presented and discussed in this paper. ARGOS is a joint development of GKSS/Research Centre Geesthacht and DASA/MBB, Space Communication and Propulsion Systems Division in Ottobrunn, Germany.

Being well suited for remote detection of hazardous organic gases, CO₂ laser DIAL may find its application in environmental sensing and industrial monitoring. To obtain a detailed performance picture of small mobile CO₂ DIAL sensors, simulations of aerosol backscatter signals are performed.

Damaged spruce needles showed a smaller rise from ground level fluorescence (Fo) to maximum fluorescence (Fm) of the fast component of the Kautsky effect and a corresponding smaller decline (Fd) to the steady state fluorescence (Fs) of the slow component of the Kautsky effect relative to healthy needles. Fm/Fo and (Fm - Fs)/Fs or RFd derived photosynthesis indices were also correspondingly lower for metal-stressed versus nonstressed needles, and indicate a disfunctioning of the photosynthetic apparatus in the damaged needles and a consequent decrease in their photosynthetic capacity. A strong linear relationship occurs between the Fm/Fo indices of the fast component of the Kautsky effect and needle chlorophyll a content, although less strong relationships occur with respect to chlorophyll b and a + b contents, as well as between the RFd indices of the slow component of the Kautsky effect ad needle chlorophyll a, b, and a + b contents. The possibility exists to utilize chlorophyll fluorescence induction kinetics as a fast and reliable means to predict the chlorophyll content of plant foliage, in addition to providing an indicator of plant vitality.

Chlorophyll fluorescence induction kinetics were used to assess different features of the photosynthetic system of trees affected by forest decline and of control trees. Several parameters can be derived from the kinetics: F_o and F_o', the initial fluorescence intensity and its initial slope, the maximum intensity F_p, and the final steady- state intensity F_s. Furthermore, the ratio R(t) of the two fluorescence components at 685 nm and 730 nm shows the initial value R_o, the minimum value R_m, and the steady- state value R_s. Measurements were carried out on spruce, birch, oak, and poplar. If one considers the variation of the parameters obtained from the different tree species and the effect on the parameters due to the forest decline, the parameters R_o and F_p/F_o exhibit a maximum capability to differentiate between control trees and those affected by forest decline.

Attention is given to airborne laser bathymetry, a new tool which allows very rapid hydrographic surveys to be conducted in shallow waters without the common marine navigation difficulties. A pulsed laser carried on board a plane or a helicopter sends an ultrashort luminous pulse to the sea bed. This pulse is located in the blue-green spectrum where sea water is the least absorptive. The pulse propagates with practically no attenuation to the surface of the water, where a first echo is retransmitted to the receiver due to the difference between the air and water index. When the pulse passes through the water, the suspended particles of organic origin (plankton) or mineral origin (sediment) diffuse the light transmitted and produce a diffused echo along the water column. On contact with the sea bed, a final echo, the amplitude of which will depend on the bottom type, is transmitted. Laser bathymetry is expected to reduce survey cost by a factor of 3. The instrument's mobility implies that areas far away from one another can be surveyed on the same day.

After a brief comparison between acoustics and optics in underwater imaging and a short description of optical propagation in scattering media, different methods of imaging with a blue-green laser source are described. As examples, description of two experimental mock-ups are shown: point by point scanning imager and global TV. Possible enhancement by polarimetric analysis is mentioned. The main technical constraints for operational lidar imagers are further discussed.

A time resolved lidar fluorosensor operated from a helicopter is described. This sensor is particularly suitable for oil identification in a slick on the sea surface and to perform accurate measurements on the water beam attenuation coefficient in addition to the evaluation of the dissolved organic matter (DOM) content in the water column. The procedure to evaluate these parameters is presented and discussed. Two validation campaigns were performed over the Ligurian Sea in July and over the Adriatic Sea in December 1991. The results conformed to what was expected from the simulation experiments. In particular, the possibility of measuring the water beam extinction coefficient at the excitation wavelength (355 nm) and at the water Raman wavelength (404 nm) from the time dependence of the backscattering and Raman signals of a single shot was demonstrated. It was therefore possible to draw profiles of the water transparency and of the DOM fluorescence intensity along the flight line.

An experimental evaluation of an airborne depth sounding lidar called FLASH (FOA Laser Airborne Sounder for Hydrography) is presented. The lidar is based on a scanning frequency doubled Nd-YAG laser and is borne by a helicopter. An example of measured waveforms is compared with those obtained by analytical and Monte Carlo modeling.

The results of remote field diagnostics of brackish-water phytoplankton by tunable lidar on board a research vessel are presented. The authors have developed a study approach based on the spectral signature method and the method of selective excitation of phytoplankton pigments to detect pigment composition of phytoplankton communities in vivo.

Field measurements of the spatial distribution of phytoplankton by tunable lidar on board a research vessel are reported. The possibility of applying laser remote sensing to the diagnostics of hydrophysical processes in the upper layers of the sea is discussed. Twelve tracks in different directions were sensed. Marked periodical structures were observed when the vessel was moving at small angles in the direction of a swell. The frequencies of periodic structures correlated with the angle between the vessel's motion and swell directions. When this angle was increased, the frequency increased proportionally. At a right angle, the periodic structures disappeared. The results do not contradict the hypothesis of the influence of internal waves on the spatial distribution of phytoplankton.

Fiberoptical sensors are a promising analytical tool for on line and in situ detection of trace contaminants in freshwater ecosystems. The described fiber optical sensor is based on time- resolved, laser-induced fluorescence and allows the detection of PAHs in the ng 1^-1 range with a fiber length of 50 m. The application of chemometrical methods to the large multidimensional spectra permits the extraction of the temporal and spectral characteristics of PAHs from multicomponent mixtures.

A ground-based Doppler wind lidar for tropospheric wind speed measurement is being developed for environmental applications such as pollutants distribution studies and climatology investigations preceding the choice of sites for power plants and factories. This paper gives a short overview on specifications and design concepts of the system and reports the obtained results about the developing and testing of the pulsed CO₂ TEA laser source. The lidar system is scheduled to be ready for laboratory measurement next year and for field use within a couple of years.

WIND is a joint project between France (CNRS-CNES-Meteo France) and Germany (DLR) to develop an airborne wind Doppler lidar for meteorological applications. The instrument specifications are derived from the measurement objectives as well as the state-of-the-art in technology. Presently an operational airborne wind lidar can be designed around the CO2 laser technology, heterodyne detection, and a conical scanning of the lidar line-of-sight to sample the atmospheric wind field. The 10-micron spectral domain is suitable for long range measurements for it corresponds to an atmospheric window and an adequate backscatter coefficient in the troposphere. The first flights are scheduled early 1995 on board the Falcon 20 operated by DLR.

The need to measure the background wind in the height range 25-60 km where there was no possibility to monitor adequately the wind field, led to the development of a new Doppler lidar designed to cover this altitude range and therefore it had to rely on Rayleigh scattering. The light source is a pulsed monomode doubled Nd:YAG laser (532 nm). The Doppler shift of the backscattered echo is measured by inter-comparing the signal detected through each of the two band-passes of a single dual high-resolution Fabry Perot interferometer tuned on either side of the backscattered line. Wind profiles extending up to about 45 km have been obtained with a preliminary device for typically 2-3 hours integration time and 2 km height resolution. The wind profiles were originally limited downwards to altitudes where the contribution of the Mie scattering by aerosols can be neglected. The possibility to adapt the instrument for wind measurements in regions where the Mie scattering becomes important has been recently theoretically and experimentally demonstrated.

Knowledge of deformation of every point of a wave front over time allows statistical turbulence parameters to be analyzed, and the definition of real time adaptive optics to be designed. An optical instrumentation was built to meet this need. Integrated in a compact enclosure for experiments on outdoor sites, the CASOAR allows the deformations of a wave front to be measured rapidly (100 Hz) and with accuracy (1 deg). The CASOAR is an active system: it includes its own light source (CW CO2 laser), making it self-contained, self-aligned and insensitive to spurious light rays. After being reflected off a mirror located beyond the atmospheric layer to be analyzed (range of several kilometers), the beam is received and detected by coherent mixing. Electronic phase is converted in optical phase and recorded or displayed in real time on a monitor. Experimental results are shown, pointing out the capabilities of this device.

We have elaborated and experimentally verified a new method of nondestructive long-range measurement of the parameters of the atmospheric turbulence with a CW Doppler lidar. The method is based on certain processing of the heterodyne Doppler spectra. In addition to the wind direction and magnitude the method yields the value of the velocity structure coefficient. The method has been experimentally verified. The measurements of structure function constant were carried out in parallel with the Doppler lidar and with the equipment (rumbo-anemometers) of the High Meteorological Mast in Obninsk (Russia). The results are in a good agreement.

Lidar measurements of geometrical (height, thickness) and radiative properties (extinction and backscattering) of the volcanically perturbed stratospheric aerosol layer are presented and discussed. The layer is located between about 12 and 25 km height. Since December 1991 the particle optical depth is between 0.1 and 0.2 for the wavelength of 0.5 micron. The determined extinction-to-backscatter, or lidar, ratios of 10 to 40 sr indicate an increase of the mean radius of the stratospheric sulfuric acid droplets compared to the respective value for the background distribution.

An infrared aerosol lidar has been developed which operates at the eyesafe wavelengths 1.54 or 1.56 micron, generated by a Nd:YAG laser and stimulated Raman scattering. The pulse energy is greater than 30 mJ at 10 Hz repetition rate. The backscattered radiation is collected by a 38 cm F/5 Newtonian telescope with a f = 9 mm, 84-deg field-of-view eyepiece, and detected by a cooled germanium detector. Signal digitizing is done by a 12-bit 10-MHz transient recorder and a PC 386. After assembling of the system Raman-shifting experiments were carried out to optimize the output in the eyesafe wavelength. First backscatter profiles are expected to be available at the conference.

Lidar is the short form of light detection and ranging. The first application of a lidar system was, as in the radar technique, the determination of the distance to large-sized particles (target recognition). Nowadays, it is of more interest to measure the structure of the atmosphere in far distances (remote sensing) to get, for example, information about the mass concentration of the industrial pollution or the visibility conditions in dense fog. In this case the action and reaction of the laser light with the particles is made by very small and different scatterers (molecules, atoms, or aerosols) and, therefore, extremely complex. A simulation program that helps to determine the visibility with a lidar has been developed to present the effects of the components of the system (laser, transmitter, receiver) as well as the parameters of the atmosphere (inhomogeneities, fog, clouds) in a convenient way. A change in any parameter is taken into account instantaneously, so this program can be called an almost real time simulator. A computer with a graphic user interface was chosen to realize this as simply as possible: The Commodore Amiga. The simulation is written in `C' to get the best performance for the calculations.

Consideration is given to a novel inversion algorithm to determine the aerosol size distribution from lidar signals obtained at several wavelengths. This algorithm is based on a nonlinear fit of the backscattered measurements using a set of predetermined functions. Size distribution profiles of tropospheric aerosols in different meteorological conditions - clear, hazy, and cloudy atmosphere - are determined on the basis of lidar measurements made in central Switzerland, which is characterized by a specific microclimate. Measurements of stratospheric clouds and aerosols created by the Pinatubo eruption were measured above Berlin and Sodankyla, Finland. These measurements are used to characterize the size distribution of these volcanic aerosols in order to estimate their influence on radiative transfer.

The possibilities for deriving optical and microstructural characteristics of atmospheric aerosols by spectral polarization laser probing have been analyzed. Information about these parameters is determined by the data set of multiple wavelength laser sensing and that of polarization probing signals. This paper presents technical characteristics and structures of multifunctional lidar systems. These systems provide atmospheric probing in a wide spectral range and measuring polarization properties of reflected signals. The experimental results of tropospheric and stratospheric spectral polarization lidar measurements are generalized.

A combined ozone and aerosol LIDAR has been developed at the DLR Institute of Atmospheric Physics in Oberpfaffenhofen, Germany. It is an airborne version that permits the recording of two-dimensional backscatter profiles at three wavelengths (532 nm, 354 nm, and 308 nm). From these profiles it is possible to calculate the spatial distribution of stratospheric aerosols, PSCs, and ozone. For the ozone retrieval the DIAL-principle is applied to the signals at the two shorter of the above mentioned wavelengths. The first mission with this system was performed during the EASOE campaign last winter.

The Rayleigh lidar concept is based on the measurement of Rayleigh scattering when a laser beam is sent in the atmosphere. The principle of such a detection by Rayleigh scattering was developed thanks to the CNRS team of Service d'Aeronomie (M. L. Chanin and A. Hauchecorne). The Rayleigh lidar provides the spacial and temporal atmosphere density and temperature information which have a direct impact on the space device trajectory. This measurement can be obtained thanks to balloon probes or radar up to 30 km in height. The Rayleigh lidar enables these measurements to be made continuously, up to 90 km in height. It can be used for several applications such as: space device assistance, constitution of the statistical data bank of the atmosphere, and the study of physical phenomena in the atmosphere. The new SESO Rayleigh lidar system `LIRA' is transportable and commercially available. Description, characteristics, and results are presented.

The paper describes a novel lidar method to measure the temperature between 5 and 30 km independently of the presence of aerosols, either because of their spectral contribution or their effect on extinction. The method is based on an idea proposed by Cooney (1972), which takes into account the temperature dependence of the Raman spectrum, and which was later used to measure the temperature between 0 and 2 km of altitude using the rotational Raman spectrum backscattered by molecular nitrogen. It is shown that, under certain instrument conditions, the function which makes it possible to calibrate the lidar does not depend on pressure, which implies that the same function can be used at all altitudes, without causing an unacceptable error in temperature measurement. Results obtained with this technique agree with radiosondes within 0.5 K between the ground and 20-25 km.

An airborne near infrared differential absorption lidar (DIAL) has been completed for meteorological applications. This system is based on a Nd:YAG pumped narrow-band tunable dye laser for both the on- and off-line measurements. Performing H₂O measurements within and above the planetary boundary layer (PBL) up to an altitude of 4 km, it successfully participated in the European Field Experiment on Desertification Threatened Areas (EFEDA '91) conducted in Spain in the summer of 1991. Data processing of the lidar signals provides range resolved horizontal and vertical water vapor profiles, horizontal power spectra of turbulence, and aerosol backscattering profiles. Water vapor profiles are being calculated using gliding averages of single lidar returns. Typical horizontal resolutions range from 1.3 to 3 km with vertical resolutions varying from 300 to 600 m, depending on the signal-to-noise ratio, in order to meet a 5 to 10% accuracy. The systematic errors, however, are estimated to be around 6%. The vertical water vapor profiles agree well with radiosonde measurements.

Space-based measurements of water vapor, temperature, and wind velocity conducted with Tm,Ho:YAG technology emitting at 2 microns are reported. Emphasis is placed on an investigation of spectral lines, laser transmitter and receiver requirements, and parametric studies leading to the final instrument design. Transmitter and receiver requirements, and mass, size, and power budgets for the three selected lidar concepts (incoherent DIAL, coherent DIAL, and coherent Doppler wind lidar) are presented in tabular form.

A high energy flashlamp-pump Ti:sapphire laser has been developed. Pulse energies up to 1200 mJ have been obtained at a repetition rate of 10 Hz. Laser tuning (700 nm to 1050 nm) is achieved by a 4-plate Lyot-filter, with a bandwidth of 0.3 nm. Q-switching has also been performed to improve the temporal (spatial) resolution, with 300 mJ output in 20 nS. These unique specifications are extremely attractive for lidar applications. A flashlamp-pumped Ti:sapphire based lidar system has been used for several multispectral investigations such as measurements of volcanic aerosols and PSCs in the polar stratosphere during the EASOE campaign. Promising applicataion of Ti:sapphire lasers, like measurements of humidity in the troposphere and the stratosphere, are presented. UV-DIAL applications for pollution monitoring using frequency-doubling-tripling or mixing are also discussed.

A development activity is in progress to renew the ENEL DIAL with the aim of obtaining a reliable multipurpose system, able to control different pollutants (SO₂, NO, NO₂, O₃, Hg, CO, etc.), together with important meteorological parameters such as water vapor concentration and temperature. The system is based on an Nd:YAG pumped Titanium:Sapphire laser, with the possibility to be tuned from UV to IR by frequency mixing. The frequency control is obtained by means of stabilized laser diode injection into the Titanium:Sapphire laser. The Nd:YAG pump laser, designed for spaceborne use in the framework of an ESA contract, is a very compact device, based on a master oscillator plus amplifier (MOPA) configuration with a Gaussian resonator. The main feature of the transient recorder is the high dynamic range (18 bit, i.e., dynamic range of 250,000) with a 10 MHz sampling rate and a reduced jitter for the laser pulse emission (less than 10 ns). The receiver photomultiplier follows a quadratic gain versus range law in order to reduce the LIDAR signal dynamic range. Most of the work concerned with these critical LIDAR components (laser, wavelength control, receiver) is presented.

The concept of a receiver technique for high spectral resolution lidars is presented. The optical amplifier placed in front of the detector acts like an active narrow-band interference filter. The wavelength is selected by the tuning element of the amplifier and, therefore, the application to a variety of lidars is possible. The application can be an incoherent Raman lidar as well as short wavelength Doppler lidars with direct detection. The advantages and disadvantages of optical amplifiers as preamplifiers are discussed.

The troposphere is such a complex system that the prediction of air pollution scenarios often requires the use of sophisticated model calculations. Such models contain emissions, chemistry, transport (meteorology), and deposition processes. A large fraction of the information on the detailed processes which are put into the model, in particular where the chemistry is concerned, comes from laboratory measurements of homogeneous and inhomogeneous elementary reactions. The model is then tested against in situ measurements in the real troposphere. One of the key species in tropospheric chemistry is the OH radical which is an aggressive oxidizing agent. Due to its high reactivity, its concentration in the troposphere is low, generally below 5 X 10⁶ radicals per cm³. This remains difficult to measure in general, although under optimal conditions long path optical absorption, as well as laser induced fluorescence (LIF) techniques, have now been successfully applied. If one wishes to investigate the reaction of, for example, OH radicals with molecule X, one approach called flash photolysis is often used in laboratory studies. A very high and easily observable concentration of OH radicals is created by a photochemical pathway using a short flash of light. The decay of the OH concentration to its equilibrium value in the presence of a large excess of X is studied in the time domain. This observation permits the determination of the rate constant k for the reaction OH + M yields products. We would like to test if our understanding of OH reactions in the atmosphere is sufficiently complete under many types of atmospheric conditions. The comparison of model calculations of OH concentrations with measurements is one solution to this problem, which, however, can not always be applied. Hence, we have proposed an alternative solution in which essentially a flash photolysis experiment is done in situ in the troposphere. Using a short laser pulse, for example a high concentration of OH radicals is created by photodisassociating O₃ in the presence of water molecules: O₃ + hv yields^k O(¹D) + O₂. O(¹D) + H₂O yields 2OH. We then follow the decay of the OH concentration by different spatially resolved techniques like laser-induced fluorescence (LIF) or DIAL (differential absorption lidar). This is then done under different conditions (clean troposphere, polluted troposphere) where the significant chemical and meteorological parameters have been separately measured, so that model calculations can be compared with the time dependent OH concentration. In case of disagreement, the model is presumed to be incomplete and other terms must be added like additional homogeneous or inhomogeneous reactions of OH. We have termed this novel approach to in situ measurement of tropospheric chemistry pump-and-probe lidar. Clearly the technique is general and is not limited to the generation and/or detection of high concentrations of OH radicals, as is shown. In the present description we present a computer simulation of some chemical scenarios, in order to obtain some preliminary information on what can be learned from pump-and-probe lidar experiments. The emphasis is on the chemical kinetics. Transport is taken into account in a future paper. Hence, the scenarios, within the limits of the simplified chemical model, are only realistic for a static atmosphere without turbulence or diffusion, i.e., at short delays after the laser flash perturbation.

In order to increase the spatial resolution of a lidar system without shortening the laser pulses a new signal evaluation method procedure is proposed. The convolution type lidar equation is approximated by a system of linear equations which can easily be solved. For reducing the noise the evaluation of a series of pulses is investigated. A numerical simulation of the proposed scheme is presented which clearly shows the increase of spatial resolution.

The lidar systems, which are working as a registrar of single pulses, can use several kinds of multicanal analyzers. These multicanal analyzers have universal applications and limit the possibilities of the lidar measurements. The electron system for lidar sensing of the phenomena allows an interruptable registration of single pulses for a long time. Such a system can allow preliminary processing of the signal. The results are sent to the microcomputer via standard interface bus.

A new detection method is presented for detecting the weak optical signal scattered back from a topographic target. The weak signal arriving from a topographic target is amplified by the frequency modulated transmitter laser of the lidar system and the amplified signal is mixed with the frequency shifted transmitter laser beam. The concept is used for a differential absorption lidar and excellent common mode rejection has been achieved. Field applications of the system are presented.

The use of lidar systems as distance noncontact devices by the control of the parameters of phenomena for ecological protection needs a development of technical capabilities for the realization of that idea. The high dynamics of the processes in the phenomena can be measured only with the real-time data acquisition system. The real-time working system allows an express analysis of lidar information. The lidar signal includes information of the parameters of the ecological pollution. In such systems as a control device are personal microcomputers, with special software. The stream of the information into the lidar systems usually needs two or more input canals and have an input speed about 20 Mbytes/canal. It is difficult for the microcomputers to collect this data directly, without additional devices.

New methods for research and development need the use of microcomputer equipment to design microcomputer controlled data acquisition systems for measurement of the parameters of the phenomena, processing of the received data, and visualization of the results. The microcomputer, which can be used as a controller of the devices, offers the possibility to combine several separate working instruments and devices into a system with various applications via an interface bus like GPIB.